If you just spam something like ice > water > fusion to end in a lv3 SC, you looking at 4761 spheres of each type, at 100k/fusion and 10k/lv1 sphere, its 476100000 + 95220000 (fusion and the T1 spheres together), im not adding possible losses or HQs to the equation because factoring those 2, you wont get these numbers moved much.
You looking at 571M in spheres this way plus catalysts which will be 38m for the T2 catalysts and around 3m for all the T1 ones, adding to a total of 612M~
If you dont wanna get bored and can buy fire/thunder/frag spheres at the same T1/t2 prices as i posted, its the same price in the end but you can change skillchain so make it a bit less annoying.
Personally i like ending in double light/dark for speed but if you wanna save money, this isnt a bad way to do it imo.